1. Field of the Invention
This invention relates to a connection processing method for radiation images and a radiation image processing apparatus for carrying out the method. This invention particularly relates to connection processing for radiation images, which is performed in cases where a radiation image of an object having been recorded on a plurality of stimulable phosphor sheets associated with one another is to be reconstructed.
2. Description of the Prior Art
Recently, as systems capable of obtaining radiation images recorded even when energy intensity of radiation, to which a recording medium is exposed, varies over a wide range, computed radiography systems (CR systems) have widely been used in practice. With the CR systems, a radiation image of an object, such as a human body, is recorded on a stimulable phosphor sheet. The stimulable phosphor sheet, on which the radiation image has been stored, is then exposed to stimulating rays, such as a laser beam, which cause it to emit light in proportion to the amount of energy stored thereon during its exposure to the radiation. The light emitted by the stimulable phosphor sheet, upon stimulation thereof, is photoelectrically detected and converted into an image signal. The image signal is then processed and used for the reproduction of the radiation image of the object as a visible image on a recording material.
In the CR systems, stimulable phosphor sheets having various different sizes, such as a 14xe2x80x3xc3x9717xe2x80x3 size, a 14xe2x80x3xc3x9714xe2x80x3 size, a 10xe2x80x3xc3x9712xe2x80x3 size, and a 8xe2x80x3xc3x9710xe2x80x3 size, have heretofore been used in accordance with the objects whose images are to be recorded. However, in the fields of the orthopedic surgery, for the purposes of measuring the degree of bending of the spinal column, and the like, there is a strong demand for the use of a long image ranging from a pattern of the neck to a pattern of the waist as a single image. Therefore, it has been studied to utilize stimulable phosphor sheets which are longer than the aforesaid sizes in a predetermined direction.
However, in cases where the long stimulable phosphor sheets are utilized, designs of radiation image read-out apparatuses for reading out the radiation images from the stimulable phosphor sheets, such as the designs of sheet conveyance paths in the radiation image read-out apparatuses, must be altered markedly so as to be adapted to the long stimulable phosphor sheets. The radiation image read-out apparatuses must thus be designed for the exclusive use for the long stimulable phosphor sheets. Therefore, the problems occur in that the radiation image read-out apparatuses designed for the long stimulable phosphor sheets are disadvantageous in the aspect of cost.
Accordingly, a technique may be utilized, wherein two stimulable phosphor sheets having the conventional sizes are associated with each other to form an apparently long stimulable phosphor sheet, a long image is recorded on the apparently long stimulable phosphor sheet, and thereafter the two stimulable phosphor sheets constituting the apparently long stimulable phosphor sheet are subjected to image read-out operations one after the other. With the technique, the image read-out to operations can be performed by utilizing the conventional radiation image read-out apparatus without its design being altered, and the problems described above do not occur.
Also, with the technique described above, three or more stimulable phosphor sheets can be associated with one another to form an apparently long stimulable phosphor sheet, and a long image of an object can be recorded on the apparently long stimulable phosphor sheet. Also, a plurality of stimulable phosphor sheets can be associated with one another in two axis directions, which are normal to each other, in order to form an apparently wide, long stimulable phosphor sheet, and a wide, long image of an object can be recorded on the apparently wide, long stimulable phosphor sheet. Therefore, the technique described above has good adaptability to objects.
In cases where at least two stimulable phosphor sheets are associated with each other to form an apparently long stimulable phosphor sheet and an image of an object is recorded on the apparently long stimulable phosphor sheet, if the two adjacent stimulable phosphor sheets among the plurality of the stimulable phosphor sheets are considered, the two adjacent stimulable phosphor sheets may be associated with each other such that their edges are in abutment with each other. Alternatively, the two adjacent stimulable phosphor sheets may be associated with each other such that portions of the two sheets overlap each other. However, with the technique wherein the two adjacent stimulable phosphor sheets are associated with each other such that their edges are in abutment with each other, loss of image information will inevitably occurs at the boundary area between the two adjacent stimulable phosphor sheets. With the technique wherein the two adjacent stimulable phosphor sheets are associated with each other such that the portions of the two sheets overlap each other, such loss of image information does not occur.
In cases where a radiation image of an object is recorded on the two adjacent stimulable phosphor sheets, which are associated with each other such that the portions of the two sheets overlap each other, and the two radiation images having been read out from the two stimulable phosphor sheets are connected with each other, as for the overlapping regions of the two radiation images, which overlapping regions correspond to the overlapping areas of the two stimulable phosphor sheets, the image information within the overlapping region of the radiation image having been read out from the stimulable phosphor sheet located on the side close to the object should preferably be employed for the reasons described below.
Specifically, in cases where radiation carrying image information of the object is irradiated to a first stimulable phosphor sheet, which is one of the two adjacent stimulable phosphor sheets associated with each other in the manner described above and which is located on the side remote from the object, and the other second stimulable phosphor sheet, which is located on the side close to the object, the overlapping area of the first stimulable phosphor sheet, upon which the overlapping area of the second stimulable phosphor sheet overlaps, is exposed to the radiation having decayed to a dose smaller than the dose of the radiation irradiated to the other area of the first stimulable phosphor sheet, which area does not overlap the second stimulable phosphor sheet. Therefore, the image density of the overlapping region of a first radiation image having been read out from the first stimulable phosphor sheet becomes lower than the image density of the non-overlapping region of the first radiation image. Accordingly, if the image information recorded within the overlapping region of the first radiation image is employed with respect to the overlapping regions of the first radiation image and a second radiation image, which has been read out from the second stimulable phosphor sheet, and a radiation image is thereby reconstructed from the first and second radiation images, a reconstructed radiation image will be obtained in which the image density of the long, narrow overlapping region is lower than the image density of the other region. As a result, a reconstructed radiation image, which has good image quality and can serve as an effective tool in, particularly, the efficient and accurate diagnosis of an illness, cannot be obtained. However, as for the second radiation image having been read out from the second stimulable phosphor sheet, the image density of the overlapping region of the second radiation image is identical with the image density of the non-overlapping region of the second radiation image. Therefore, in cases where the image information recorded within the overlapping region of the second radiation image is employed with respect to the overlapping regions of the first radiation image and the second radiation image, and a radiation image is thereby reconstructed from the first and second radiation images, the problems do not occur in that a reconstructed radiation image is obtained in which the image density of the long, narrow overlapping region is lower than the image density of the other region.
However, for reasons of constitution of a radiation image read-out apparatus for reading out a radiation image from a stimulable phosphor sheet, it often occurs that the image information recorded at an edge area of the stimulable phosphor sheet cannot be read out. In such cases, of the image information recorded within the overlapping region of the second stimulable phosphor sheet, which image information is to be employed for reconstructing a composite radiation image, the image information recorded at the unreadable edge area of the second stimulable phosphor sheet cannot be read out. Therefore, if the composite radiation image is reconstructed from the first and second radiation images having thus been read out from the first and second stimulable phosphor sheets, a composite radiation image will be obtained in which the image information corresponding to the unreadable edge area has been lost. Accordingly, as for the part of the overlapping region corresponding to the unreadable edge area, the image information, which has been read out from the corresponding part of the overlapping area of first stimulable phosphor sheet and which has a low image density must be employed.
In such cases, since the image density of the overlapping region of the first radiation image having been read out from the first stimulable phosphor sheet is lower than the image density of the other non-overlapping region of the first radiation image, the problems occur with the reconstructed radiation image in that a long, narrow area having a low image density remains within the overlapping region.
The primary object of the present invention is to provide a connection processing method for radiation images, wherein a single radiation image is reconstructed from radiation images having been read out from a plurality of stimulable phosphor sheets, which are associated with one another with portions of two adjacent stimulable phosphor sheets overlapping each other and on which the radiation images have been recorded, such that, even if image information recorded at an edge area of one of the two adjacent stimulable phosphor sheets cannot be read out and is therefore lost in the radiation image having been read out from the one stimulable phosphor sheet, an area having a low image density, which adversely affects the image quality of the reconstructed radiation image and its capability of serving as an effective tool in, particularly, the efficient and accurate diagnosis of an illness, will not be formed at part of an overlapping region of the reconstructed radiation image, which overlapping region corresponds to overlapping areas of the two adjacent stimulable phosphor sheets.
Another object of the present invention is to provide a radiation image processing apparatus for carrying out the connection processing method for radiation images.
In a connection processing method for radiation images and a radiation image processing apparatus in accordance with the present invention, in cases where at least a certain area within an overlapping region of a first radiation image having been read out from a first stimulable phosphor sheet located on the side that is remote from an object at an area at which two adjacent stimulable phosphor sheets overlap each other, is employed in image reconstruction with respect to overlapping regions of two radiation images having been read out from the two adjacent stimulable phosphor sheets, which overlapping regions correspond to overlapping areas of the two adjacent stimulable phosphor sheets, an image density correction is made such that the image density of the area of the first radiation image, which area is employed as at least the certain area within the overlapping region of the radiation image, approximately coincides with the image density of a non-overlapping region of the first radiation image other than the overlapping region and/or the image density of a second radiation image having been read out from a second stimulable phosphor sheet, which is located on the side close to the object.
Specifically, the present invention provides a connection processing method for radiation images, in which a single radiation image of an object is recorded on a plurality of stimulable phosphor sheets associated with one another such that portions of two adjacent stimulable phosphor sheets overlap each other, and in which connection processing is performed on a plurality of radiation images having been read out from the plurality of the stimulable phosphor sheets respectively, such that the single radiation image of the object is reconstructed from the plurality of the read-out radiation images, the method comprising the steps of:
i) as for at least a certain area within overlapping regions of two radiation images having been read out from the two adjacent stimulable phosphor sheets, which overlapping regions correspond to overlapping areas of the two adjacent stimulable phosphor sheets, employing an area within the overlapping region of a first radiation image having been read out from a first stimulable phosphor sheet located on the side remote from the object, which first stimulable phosphor sheet is one of the two adjacent stimulable phosphor sheets, and
ii) making an image density correction such that an image density of the area within the overlapping region of the first radiation image, which area is employed as at least the certain area within the overlapping regions of the two radiation images having been read out from the two adjacent stimulable phosphor sheets, approximately coincides with the image density of a non-overlapping region of the first radiation image other than the overlapping region and/or the image density of a second radiation image having been read out from a second stimulable phosphor sheet, which is located on the side close to the object.
The term xe2x80x9cat least a certain area within overlapping regions of two radiation imagesxe2x80x9d as used herein means, for example, the area corresponding to the unreadable edge area of the second radiation image having been recorded on the second stimulable phosphor sheet.
The term xe2x80x9cside remote from an objectxe2x80x9d as used herein means the side remote from the object at the area at which the two adjacent stimulable phosphor sheets overlap each other.
The term xe2x80x9cimage densityxe2x80x9d as used herein means the gray level, the luminous level, and the like, in an image having gradation. The term xe2x80x9cimage densityxe2x80x9d as used herein also embraces the meaning as the luminance of an image displayed on a display device, such as a cathode ray tube (CRT) display device. In cases where the radiation image is expressed as an image signal, the term xe2x80x9cimage densityxe2x80x9d as used herein represents the image signal value.
The correction of the image density (or the luminance, or the like) may be made before the radiation image is reconstructed. Alternatively, the image density correction may be made after the radiation image has been reconstructed. In cases where the image density correction is made before the radiation image is reconstructed, the image density correction should preferably be made with respect to the entire area of the overlapping region of the first radiation image. In such cases, the processing for the image density correction can be kept simpler than when only the area of the first radiation image to be utilized for the image reconstruction is extracted and the image density correction is made with respect to the extracted area.
The image density correction may be made with one of various techniques. For example, the image density may be shifted uniformly (i.e., a predetermined image density shift value may be added to the image density to be corrected). Alternatively, the image density may be multiplied by a predetermined value (i.e., a predetermined image density shift factor). The predetermined value for the shifting and the predetermined value for the multiplication can be determined in accordance with, for example, the mean value of the image density values of the readable area of the overlapping region of the second radiation image, which was able to be read out as the image information of the overlapping region (i.e., the area of the overlapping region other than the unreadable edge area whose image information was lost in the image read-out operation), and the mean value of the image density values of the area of the first radiation image, which area corresponds to the readable area of the overlapping region of the second radiation image.
Also, as a specific technique for the image density correction, different image density correcting processes should preferably be performed for (a) a boundary-neighboring subarea, which neighbors with a boundary between the overlapping region and the non-overlapping region of the first radiation image (and contains the boundary), the boundary-neighboring subarea being located in the area within the overlapping region of the first radiation image, which area is employed as at least the certain area within the overlapping regions of the two radiation images having been read out from the two adjacent stimulable phosphor sheets, and (b) the subarea other than the boundary-neighboring subarea, the other subarea being located in the area within the overlapping region of the first radiation image, which area is employed as at least the certain area within the overlapping regions of the two radiation images having been read out from the two adjacent stimulable phosphor sheets. Specifically, the overlapping region and the non-overlapping region of the first radiation image have been recorded with different doses of radiation, and therefore a boundary line image pattern due to the difference in image density is formed at the boundary between the overlapping region and the non-overlapping region of the first radiation image. The boundary line image pattern has a low sharpness due to effects of scattering of the radiation, and the like. In such cases, if the image density correction for uniformly setting the image density at a high value is performed on the low image density side with respect to the boundary line image pattern (i.e., on the overlapping region), the image density of the area neighboring with the boundary line image pattern will become higher than the image density of the non-overlapping region, and an artifact will occur.
Therefore, with respect to the boundary-neighboring subarea, an image density correcting process different from that for the subarea other than the boundary-neighboring subarea is performed. In this manner, the artifact described above can be prevented from occurring.
Specifically, the image density correcting process for the subarea other than the boundary-neighboring subarea should preferably be performed by the addition of a predetermined image density shift value regardless of a distance from the boundary or by the multiplication of a predetermined image density shift factor regardless of the distance from the boundary, and the image density correcting process for the boundary-neighboring subarea should preferably be performed by the addition of image density shift values changing in accordance with the distance from the boundary or by the multiplication of image density shift factors changing in accordance with the distance from the boundary.
Specifically, the term xe2x80x9cboundary-neighboring subareaxe2x80x9d as used herein means the subarea neighboring with the boundary, which subarea has the range of the width of the boundary line image pattern having become unsharp and wide for the reasons described above or has a range slightly wider than the range of the width of the boundary line image pattern.
Further, smoothing processing (such as median filtering processing) should preferably be performed with respect to the area, which has been subjected to the image density correction, and an area in the vicinity of the area, which has been subjected to the image density correction, in the radiation image after being reconstructed as the single radiation image. In cases where the smoothing processing is thus performed, even if an image pattern of a boundary line occurs due to a slight difference in image density between the area within the overlapping region of the first radiation image, which area has been subjected to the image density correction, and the readable area of the overlapping region of the second radiation image, the image pattern of the boundary line can be suppressed. Therefore, a reconstructed radiation image can be obtained, which has good image quality and can serve as an effective tool in, particularly, the efficient and accurate diagnosis of an illness.
In order for the image information recorded within the overlapping regions to be reconstructed, it is necessary for the overlapping regions to be detected. The detection of the overlapping regions may be performed with one of various techniques.
Specifically, position matching markers formed from a material having a low radiation transmittance may be located at the overlapping areas of the two adjacent stimulable phosphor sheets, and the image recording operation may be performed in this state. Thereafter, the image patterns of the position matching markers appearing within the overlapping regions of the two radiation images may be utilized as reference image patterns for the position matching, and the matching of positions of the two radiation images with each other may thereby be performed. In this manner, the overlapping regions can be detected.
Alternatively, instead of the position matching markers being utilized, a subregion within the overlapping region of the radiation image having been read out from one of the two adjacent stimulable phosphor sheets, which subregion contains a feature image pattern, may be set as a template, and template matching may be performed for searching a subregion in the radiation image having been read out from the other stimulable phosphor sheet, which subregion coincides with the template having been set. The matching of positions of the two radiation images with each other may thus be performed. In this manner, the overlapping regions can be detected.
As another alternative, the overlapping regions may be detected with a technique for detecting the boundary line image pattern having been formed between the overlapping region and the non-overlapping region on one of the two radiation images. The technique for detecting the boundary line image pattern is simpler than the technique for performing the position matching by utilizing the markers described above and the technique for performing the position matching with the template matching. Specifically, the overlapping area of the first stimulable phosphor sheet, which is located on the side remote from the object, is exposed to the radiation having decayed to a dose smaller than the dose of the radiation irradiated to the other area of the first stimulable phosphor sheet, which does not overlap the other second stimulable phosphor sheet. Therefore, an image pattern of a boundary line due to a difference in image density is formed between the overlapping area and the non-overlapping area of the first stimulable phosphor sheet located on the side remote from the object. The boundary line image pattern may be detected with edge detection processing, or the like. Also, the matching of positions of the two radiation images with each other may be performed such that the position of the edge on the side of the overlapping region of the second radiation image, which has been read out from the second stimulable phosphor sheet located on the side close to the object, coincides with the position in the first radiation image, which position is shifted from the position of the boundary line image pattern in the first radiation image into the overlapping region (on the low image density side) of the first radiation image by the distance corresponding to the length of the unreadable edge area of the second stimulable phosphor sheet. In this manner, the overlapping regions may be detected.
In the image recording operation for recording an image of an object on a stimulable phosphor sheet, radiation is produced by a radiation source as a divergent beam and is irradiated to the object. Therefore, the size of the image of the object recorded on the first stimulable phosphor sheet located on the side remote from the object and the size of the image of the object recorded on the second stimulable phosphor sheet are slightly different from each other. The image recorded on the first stimulable phosphor sheet located on the side remote from the object is larger than the image recorded on the second stimulable phosphor sheet located on the side close to the object. Therefore, in cases where there is the risk of the difference between the sizes of the two radiation images, which are to be subjected to the connection processing, adversely affecting the image quality of the composite image after being subjected to the connection processing, image size enlargement or reduction processing may be performed on the first radiation image having been read out from the first stimulable phosphor sheet located on the side remote from the object and/or the second radiation image having been read out from the second stimulable phosphor sheet located on the side close to the object, such that the sizes of the first radiation image and the second radiation image coincide with each other.
The present invention also provides a radiation image processing apparatus for carrying out the connection processing method for radiation images in accordance with the present invention. Specifically, the present invention also provides a radiation image processing apparatus, in which a single radiation image of an object is recorded on a plurality of stimulable phosphor sheets associated with one another such that portions of two adjacent stimulable phosphor sheets overlap each other, and in which connection processing means is provided for performing connection processing on a plurality of radiation images having been read out from the plurality of the stimulable phosphor sheets respectively, such that the single radiation image of the object is reconstructed from the plurality of the read-out radiation images,
wherein the connection processing means performs the connection processing such that, as for at least a certain area within overlapping regions of two radiation images having been read out from the two adjacent stimulable phosphor sheets, which overlapping regions correspond to overlapping areas of the two adjacent stimulable phosphor sheets, the connection processing means employs an area within the overlapping region of a first radiation image having been read out from a first stimulable phosphor sheet located on the side remote from the object, which first stimulable phosphor sheet is one of the two adjacent stimulable phosphor sheets, and
the apparatus further comprises correction processing means for making an image density correction such that an image density of the area within the overlapping region of the first radiation image, which area is employed as at least the certain area within the overlapping regions of the two radiation images having been read out from the two adjacent stimulable phosphor sheets, approximately coincides with the image density of a non-overlapping region of the first radiation image other than the overlapping region and/or the image density of a second radiation image having been read out from a second stimulable phosphor sheet, which is located on the side close to the object.
The radiation image processing apparatus in accordance with the present invention should preferably further comprise smoothing processing means for performing smoothing processing (such as median filtering processing) with respect to the area, which has been subjected to the image density correction, and an area in the vicinity of the area, which has been subjected to the image density correction, in the radiation image after being reconstructed as the single radiation image.
Also, in the image recording operation for recording an image of an object on a stimulable phosphor sheet, radiation is produced by a radiation source as a divergent beam and is irradiated to the object. Therefore, the size of the image of the object recorded on the first stimulable phosphor sheet located on the side remote from the object and the size of the image of the object recorded on the second stimulable phosphor sheet are slightly different from each other. The image recorded on the first stimulable phosphor sheet located on the side remote from the object is larger than the image recorded on the second stimulable phosphor sheet located on the side close to the object. Therefore, in the radiation image processing apparatus in accordance with the present invention, in cases where there is the risk of the difference between the sizes of the two radiation images, which are to be subjected to the connection processing, adversely affecting the image quality of the composite image after being subjected to the connection processing, image size enlargement or reduction processing may be performed on the first radiation image having been read out from the first stimulable phosphor sheet located on the side remote from the object and/or the second radiation image having been read out from the second stimulable phosphor sheet located on the side close to the object, such that the sizes of the first radiation image and the second radiation image coincide with each other.
As described above, the correction processing means corrects the image density of the area within the overlapping region of the first radiation image, which area is employed as at least the certain area within the overlapping regions of the two radiation images having been read out from the two adjacent stimulable phosphor sheets. The correction processing means should preferably perform different image density correcting processes for (a) a boundary-neighboring subarea, which neighbors with a boundary between the overlapping region and the non-overlapping region of the first radiation image, the boundary-neighboring subarea being located in the area within the overlapping region of the first radiation image, which area is employed as at least the certain area within the overlapping regions of the two radiation images having been read out from the two adjacent stimulable phosphor sheets, and (b) the subarea other than the boundary-neighboring subarea, the other subarea being located in the area within the overlapping region of the first radiation image, which area is employed as at least the certain area within the overlapping regions of the two radiation images having been read out from the two adjacent stimulable to phosphor sheets.
In such cases, the correction processing means should preferably perform the image density correcting process for the subarea other than the boundary-neighboring subarea by the addition of a predetermined image density shift value regardless of a distance from the boundary or by the multiplication of a predetermined image density shift factor regardless of the distance from the boundary, and should preferably perform the image density correcting process for the boundary-neighboring subarea by the addition of image density shift values changing in accordance with the distance from the boundary or by the multiplication of image density shift factors changing in accordance with the distance from the boundary.
With the connection processing method for radiation images and the radiation image processing apparatus in accordance with the present invention, the single radiation image of the object is recorded on the plurality of the stimulable phosphor sheets associated with one another such that portions of the two adjacent stimulable phosphor sheets overlap each other, and the connection processing is performed on the plurality of the radiation images having been read out from the plurality of the stimulable phosphor sheets respectively, such that the single radiation image of the object is reconstructed from the plurality of the read-out radiation images. In the connection processing, as for at least the certain area within the overlapping regions of the two radiation images having been read out from the two adjacent stimulable phosphor sheets, which overlapping regions correspond to the overlapping areas of the two adjacent stimulable phosphor sheets, the area within the overlapping region of the first radiation image having been read out from the first stimulable phosphor sheet located on the side remote from the object, is employed. Also, the image density correction is made such that the image density of the area within the overlapping region of the first radiation image, which area is employed as at least the certain area within the overlapping regions of the two radiation images having been read out from the two adjacent stimulable phosphor sheets, approximately coincides with the image density of the non-overlapping region of the first radiation image other than the overlapping region and/or the image density of the second radiation image having been read out from the second stimulable phosphor sheet, which is located on the side close to the object. Therefore, even if the image information recorded at the edge area of the second stimulable phosphor sheet cannot be read out and is therefore lost in the second radiation image having been read out from the second stimulable phosphor sheet, the problems can be prevented from occurring in that an area having a low image density, which adversely affects the image quality of the reconstructed radiation image and its capability of serving as an effective tool in, particularly, the efficient and accurate diagnosis of an illness, is formed at part of the overlapping region of the reconstructed radiation image, which overlapping region corresponds to the overlapping areas of the two adjacent stimulable phosphor sheets.